The new chairman of Japan's national broadcaster NHK has expressed regret for defending the country's use of wartime sex slaves during remarks in which he also suggested he would toe the government line on key diplomatic issues.

The furore also risks adding to the prime minister Shinzo Abe's diplomatic woes amid rising tensions with China and South Korea over territorial disputes and interpretations of Japan's wartime conduct.

Momii said brothels were "common in any country at war" at the time, and described as "puzzling" criticism of Japan's enslavement of up to 200,000 mainly Korean, Chinese and Filipino women – euphemistically known as "comfort women" – in frontline brothels across Asia between 1932 and 1945.

"Can we say there were none in Germany or France? It was everywhere in Europe," he said. "In the current moral climate the use of comfort women would be wrong. But it was a reality of those times.

"Korea's statement that Japan was the only nation [that used sex slaves] is puzzling. Give us money, compensate us, they say, but since all of this was resolved by the Japan-Korea peace treaty, why are they reviving this issue? It's strange."

Momii, the former vice-chairman of a trading house, with no previous broadcasting experience, acknowledged on Monday that his remarks had been "extremely inappropriate".

He repeated his stance that he had been speaking in a private capacity, but said: "Even as an individual opinion, it is not something I should have said. It was my first time [speaking] at such an occasion and I did not know the rules."

Momii had earlier attempted to withdraw the remarks after journalists pointed out that they were made during a press conference in his role as chairman of NHK, a hugely influential, publicly funded broadcaster.

The ruling party in South Korea called on Momii to resign and apologise to the country's people. Lee Hye-hoon, a senior figure in the Saenuri party, described the comments as "heartbreaking" at a party meeting, according to a Saenuri statement quoted by Kyodo News.

Lee said: "If there is a group of people with a conscience in Japan, how could a person like Momii, with such an unethical perception and who made such an absurd remark, become the head of the public broadcaster?"

South Korea has demanded that Japan admit it forced young women into sexual slavery during its 1910-45 occupation of the Korean peninsula, and offer an official apology and compensation.

Japan insists that all compensation claims were settled when the countries normalised diplomatic ties in 1965.

In 1993, the then chief cabinet secretary, Yohei Kono, acknowledged Japan's use of wartime sex slaves and for the first time issued an official apology.

Successive prime ministers have honoured the Kono statement, but in comments made before he became leader in December 2012, Abe suggested he wanted to withdraw it.

Officials close to Abe reacted angrily to Momii's comments, with one calling for his resignation.

"I am extremely angry because these gaffes are unthinkable for the head of a media company," the Asahi Shimbun quoted an unnamed cabinet minister as saying. "He should resign immediately."

But the chief cabinet secretary, Yoshihide Suga, defended Momii's remarks at a press conference on Monday, saying they were his personal views, the Associated Press reported.

Momii's comments echo those made last year by Toru Hashimoto, the rightwing mayor of Osaka, who sparked controversy when he said the use of wartime sex slaves helped "maintain discipline" among troops.

Momii cast further doubt on NHK's commitment to editorial independence when he said it was "only natural" for the broadcaster to side with the government on contentious issues in its international output.

Asked about Japan's dispute with China over ownership of the Senkaku islands in the East China Sea (known as the Diaoyu in China), Momii said: "International broadcasting is different from domestic. It would not do for us to say 'left' when the government is saying 'right.'"

It was "only natural to state Japan's position in no uncertain terms", he said.

Momii was seen as Abe's favoured candidate to head NHK.

Last November, Abe was accused of handpicking allies to fill several posts on NHK's board of governors, which appointed Momii this month. Momii denied he was acquainted with Abe.

It is not the first time the prime minister has been embroiled in controversy over NHK's editorial decisions. In 2005, he admitted he had urged NHK officials to alter the contents of a documentary about wartime sex slaves.

Momii also dismissed concern, not least among other media organisations, about parliament's recent approval of a state secrets law that threatens whistleblowers and journalists with imprisonment.

Now that the bill has become law, he said, there was no point in questioning it. "It would be a problem if the government's objective was the same as what concerns the public, but I don't think that is the government's purpose," he said.

Momii observed the broadcaster's legal commitment to neutrality when asked about Abe's recent visit to Yasukuni shrine, which honours 2.5 million war dead, including several class A war criminals.

"Since the prime minister made the visit based on his beliefs, that in itself is all right. I am not in a position to say whether it was right or wrong," he said. "All [NHK] can do is to simply say the prime minister visited Yasukuni."